The HPV vaccine is commonly recommended to men and women between 9 and 26. But even those who forego vaccination are seeing some benefits, new research finds.

When enough people get vaccinated for a disease, the protective benefits of the drug begin to cover even those who were unwilling or ineligible to get the shots. Even if an outbreak occurs, the logic goes, the disease can't go far because it's being contained by the surrounding vaccinated individuals. This is called "herd immunity," and it's worked beautifully for ailments like diphtheria, measles, and polio.

Now, researchers have uncovered evidence that the vaccine designed to fight human papillomavirus (HPV) is also starting to have herd immunity effects. The good news comes from scientists in Cincinnati who conducted a four-year study of sexually active women before and after vaccination.

At the beginning of the study, the researchers recruited a group of local women for analysis, none of whom had been vaccinated despite being sexually active. Two years later, in 2009, the scientists assembled a different group of women, 59 percent of whom had had at least one of the vaccine's three doses. Predictably, the vaccinated population showed a dramatic drop in the rate of HPV infection compared to the control group -- a reduction of nearly 70 percent. But even for those women who hadn't been vaccinated, the prevalence of HPV dropped by nearly half, from 30 percent to 15 percent.

The result suggests that it isn't just those who are getting the vaccine that are enjoying its benefits; it's the unvaccinated, too. It's an astonishing outcome considering that the usual threshold for herd immunity for most diseases requires 80 percent or more of the population to be vaccinated. In the case of this study, only about 60 percent of participants in the second survey had gotten shots. For HPV at least, herd protection may kick in sooner than with other diseases.

So does that mean the unvaccinated can forget about the shots altogether? Not quite. Even though it appears to be even more effective than previously thought, the treatment only protects against the four most dangerous strains of the virus and not others. In fact, while vaccine-type HPV declined in the study, the rate of infection of HPV types not covered under the vaccine actually grew from 60 percent to 75 percent. The researchers are treating that finding with some skepticism, as there's no biological reason why those strains of the virus would spread so quickly, but read broadly, the study is a ringing endorsement of HPV vaccination in general.